Exhaust device

ABSTRACT

An exhaust device for guiding exhaust gas from an exhaust pipe in front of an engine to a muffler in a rear of the engine is provided. The exhaust device includes a catalyst case in which a catalyst configured to purify the exhaust gas that passes through the exhaust pipe is accommodated, and a chamber in which a muffling chamber configured to reduce an exhaust noise is formed downstream of the catalyst case. The catalyst case is disposed in a range from a front space with respect to the engine to a front part of a lower space of the engine. The chamber is disposed so as to occupy at least a rear part of the lower space of the engine.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2021-002008 filed onJan. 8, 2021, including specification, drawings and claims isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to an exhaust device.

In the related art, as an exhaust device of a straddle-type vehicle, adevice in which a catalyst case is disposed on a lower side of an engineis known (for example, see Patent Literature 1). In the exhaust devicedisclosed in Patent Literature 1, four exhaust pipes extend downwardfrom a front of an engine, and the four exhaust pipes are integratedinto a single collecting pipe on the lower side of the engine to beconnected to the catalyst case. One exhaust pipe is connected todownstream of the catalyst case, and the exhaust pipe extends to a sideof a rear wheel and is connected to a muffler. When exhaust gas entersthe muffler from the catalyst case, the exhaust gas is expanded in amuffling chamber in the muffler, so that an exhaust noise is reduced.

-   Patent Literature 1: JP-A-2012-052514

SUMMARY

According to one advantageous aspect of the present invention, there isprovided an exhaust device, configured to guide exhaust gas from anexhaust pipe in front of an engine to a muffler in a rear of the engine,the exhaust device including:

a catalyst case in which a catalyst configured to purify the exhaust gasthat passes through the exhaust pipe is accommodated; and

a chamber in which a muffling chamber configured to reduce an exhaustnoise is formed downstream of the catalyst case, wherein

the catalyst case is disposed in a range from a front space of theengine to a front part of a lower space of the engine, and

the chamber is disposed so as to occupy at least a rear part of thelower space of the engine.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a right side view of a straddle-type vehicle according to apresent embodiment.

FIG. 2 is a front view of an engine according to the present embodiment.

FIG. 3 is a side view of the engine according to the present embodiment.

FIG. 4 is a bottom view of the engine according to the presentembodiment.

FIG. 5 is a cross-sectional view taken along a line A-A of FIG. 4.

FIG. 6 is a cross-sectional view taken along a line B-B of FIG. 4.

FIG. 7 is a bottom view of an engine according to a modification.

DETAILED DESCRIPTION OF EXEMPLIFIED EMBODIMENTS

In the exhaust device disclosed in Patent Literature 1, the lower sideof the engine is occupied by the collecting pipe and the catalyst case,and it is difficult to secure a muffling chamber with a sufficientvolume on an upstream side of the muffler. Therefore, a volume of themuffling chamber in the muffler is increased, and a size of the muffleris increased.

The present invention has been made in view of this point, and an objectof the present invention is to provide an exhaust device capable ofsecuring a muffling chamber on a lower side of an engine and reducing asize of a muffler.

An exhaust device according to one aspect of the present inventionguides exhaust gas from an exhaust pipe in front of an engine to amuffler in a rear of the engine. A catalyst case is provided downstreamof the exhaust pipe, and a catalyst that purifies the exhaust gas thatpasses through the exhaust pipe is accommodated in the catalyst case. Achamber is provided downstream of the catalyst case, and a mufflingchamber that reduces an exhaust noise is formed in the chamber. Thecatalyst case is disposed in a range from a front space to a front sideof a lower space of the engine, and the chamber can occupy at least arear part of the lower space of the engine. Since the lower space of theengine is not occupied by the catalyst case, the chamber is disposed ina wide range in the lower space of the engine, and the exhaust noise isreduced by the muffling chamber of the chamber. By securing the mufflingchamber on an upstream side of the muffler, it is possible to reduce asize of the muffler. Therefore, in a state where a muffling performanceis maintained, a risk of heat damage due to a surface of the muffler canbe reduced by reducing the size of the muffler, and a degree of freedomin designing a muffler appearance can be improved.

Hereinafter, a present embodiment will be described in detail withreference to the accompanying drawings. FIG. 1 is a right side view of astraddle-type vehicle according to the present embodiment. In thefollowing drawings, an arrow FR indicates a vehicle front, an arrow REindicates a vehicle rear, an arrow L indicates a vehicle left side, andan arrow R indicates a vehicle right side.

As shown in FIG. 1, a straddle-type vehicle 1 is formed by mountingvarious components such as an engine 30 and an electrical system on acradle type vehicle body frame 10. The vehicle body frame 10 includes amain tube 12 that extends rearward from a head pipe 11 and then bendsdownward, and a down tube 13 that extends downward from the head pipe 11and then bends rearward. A rear side of the engine 30 is supported bythe main tube 12, and a front side and a lower side of the engine 30 aresupported by the down tube 13. A fuel tank 17 is supported on the maintube 12, and a rider seat 18 and a pillion seat 19 are provided in arear of the fuel tank 17.

A pair of front forks 21 are supported by the head pipe 11 via asteering shaft (not shown) so as to be steered, and a front wheel 22 isrotatably supported on lower portions of the front forks 21. A swing arm(not shown) is swingably supported at a rear half portion of the maintube 12, and a rear wheel 23 is rotatably supported at a rear end of theswing arm. The engine 30 is connected to the rear wheel 23 via atransmission mechanism, and power from the engine 30 is transmitted tothe rear wheel 23 via the transmission mechanism. An exhaust device 50is connected to the engine 30, and exhaust gas from the engine 30 isdischarged to the outside through the exhaust device 50.

A method of expanding a catalyst capacity of a catalyst case disposed ona lower side of the engine, a method of disposing a primary catalystcase in front of the engine and disposing a secondary catalyst case onthe lower side of the engine, and the like are used in order to complywith exhaust gas regulations in recent years. In these methods, thelower side of the engine is used to expand the catalyst capacity, and itis difficult to secure a muffling chamber with a sufficient volumeupstream of a muffler unless a basic structure of the vehicle body frameand the engine is changed. Therefore, the size of the muffler isincreased, a degree of freedom in designing the muffler appearance isreduced, and an influence of the heat damage to a rider and othercomponents is increased.

Therefore, in the exhaust device 50 of the present embodiment, a frontspace of the engine 30 and a front side of a lower space of the engine30 are effectively used, and a catalyst case is compactly disposed infront of the engine 30 in comparison with a general exhaust device. Inthe lower space of the engine 30, a chamber 71 (see FIG. 3) having onlya muffling function without a catalyst is disposed, and a mufflingchamber having a sufficient volume is secured on an upstream of amuffler 82 by the chamber 71. Accordingly, it is possible to reduce asize of the muffler 82 by reducing the muffling chamber in the muffler82 while minimizing changes to the basic structure of the vehicle bodyframe 10 or the engine 30.

Hereinafter, the engine and the exhaust device will be described withreference to FIGS. 2 to 4. FIG. 2 is a front view of the engineaccording to the present embodiment. FIG. 3 is a side view of the engineaccording to the present embodiment. FIG. 4 is a bottom view of theengine according to the present embodiment.

As shown in FIGS. 2 and 3, the engine 30 is a parallel two-cylinderengine and is formed by assembling a cylinder block 32, a cylinder head33, and a head cover 34 on a crankcase 31. A drive component such as acrankshaft 35 is accommodated in the crankcase 31, and the cylinderblock 32 is attached to an upper portion of the crankcase 31. A pair ofaligned cylinder bores (not shown) disposed in a left-right direction(engine width direction) are formed in the cylinder block 32, and apiston (not shown) connected to the crankshaft 35 is disposed in eachcylinder bore. The cylinder head 33 is attached to an upper portion ofthe cylinder block 32.

A pair of intake ports (not shown) connected to the pair of cylinderbores are formed on a rear surface side of the cylinder head 33, and apair of exhaust ports 36L and 36R connected to the pair of cylinderbores are formed on a front surface side of the cylinder head 33. Thehead cover 34 is attached to an upper portion of the cylinder head 33,and a valve gear or the like is accommodated in the cylinder head 33 andthe head cover 34. An oil pan 37 that stores oil for lubrication andcooling is attached to a lower portion of the crankcase 31. An oilfilter 38 that removes a foreign matter from the oil is attached to alower portion of a front surface of the crankcase 31.

The engine 30 is assembled inside the vehicle body frame 10. The downtube 13 of the vehicle body frame 10 includes an upper down tube 14extending downward from the head pipe 11 (see FIG. 1) in a center of theengine 30 in the left-right direction, and a pair of lower down tubes15L and 15R branching left and right from a lower end of the upper downtube 14 and extending obliquely downward. The lower down tubes 15L and15R are bent rearward on the lower side of the engine 30, and are joinedto the main tube 12 at rear end portions of the lower down tubes 15L and15R. The oil filter 38 is positioned between the lower down tubes 15Land 15R.

A pair of exhaust pipes 51L and 51R extend from a front surface of thecylinder head 33 so as to avoid the upper down tube 14 and the lowerdown tubes 15L and 15R, and the exhaust device 50 that guides exhaustgas from the exhaust pipes 51L and 51R to the muffler 82 in a rear ofthe engine 30 is provided. The exhaust device 50 is provided with asmall primary catalyst 54 that functions as a starter catalyst and alarge secondary catalyst 62 that functions as a main catalyst. Theexhaust gas enters the exhaust device 50 from the exhaust ports 36L and36R, and the primary catalyst 54 and the secondary catalyst 62 purifyair pollutants such as carbon monoxide (CO), hydrocarbons (HC), andnitrogen compounds (NOx) in the exhaust gas.

In the exhaust device 50, the exhaust pipes 51L and 51R, a collectingpipe 52, a primary catalyst case 53, a bent pipe 55, a secondarycatalyst case 61, the chamber 71, an exhaust pipe 81, and the muffler 82form an exhaust passage that wraps around from the front to the lowerside of the engine 30 and extends to the rear. The exhaust pipe 51Lextends forward from the exhaust port 36L and is connected to thecollecting pipe 52, and the exhaust pipe 51R extends forward from theexhaust port 36R and then extends to a left and is connected to thecollecting pipe 52. Thus, the exhaust pipe 51L is shorter than theexhaust pipe 51R, and pipe lengths of the exhaust pipes 51L and 51R aredifferent. The exhaust pipes 51L and 51R have a circular cross section.

An upstream side of the collecting pipe 52 is bifurcated, and adownstream side of the collecting pipe 52 is formed in a cylindricalshape. The exhaust pipes 51L and 51R are connected to the upstream sideof the collecting pipe 52, and the primary catalyst case 53 is connectedto a downstream end of the collecting pipe 52. The exhaust gas thatpasses through the exhaust pipes 51L and 51R is collected by thecollecting pipe 52 and sent to the primary catalyst case 53. A firstoxygen sensor 83 is disposed between the exhaust pipes 51L and 51R on awall surface of the collecting pipe 52, and an average oxygenconcentration of the exhaust gas flowing in from the exhaust pipes 51Land 51R is detected by the first oxygen sensor 83. A detection result ofthe first oxygen sensor 83 is used for feedback control of a fuelinjection amount.

The primary catalyst case 53 is formed in a cylindrical shape, and isconnected to the collecting pipe 52 in a substantially vertical posture.Since the exhaust pipe 51L is shorter than the exhaust pipe 51R, theprimary catalyst case 53 and the collecting pipe 52 are disposed on aleft side (one side in the engine width direction) of a center line C1of the engine 30 extending in an upper-lower direction. The primarycatalyst 54 for purifying the exhaust gas that passes through theexhaust pipes 51L and 51R is accommodated in the primary catalyst case53. The primary catalyst 54 is formed by adhering a catalyst substanceto a surface of a honeycomb-shaped or lattice-shaped partition plate,and when the exhaust gas flowing in from the collecting pipe 52 passesthrough the primary catalyst 54, the air pollutant reacts with oxygenand is purified.

Since the pipe length of the exhaust pipe 51L is short, high-temperatureexhaust gas flows into the primary catalyst case 53 from the exhaustport 36L, the primary catalyst 54 in the primary catalyst case 53 iswarmed up in a short time, and a purification performance of the exhaustgas from the exhaust port 36L is improved. In this case, the pipe lengthof the exhaust pipe 51R is long and a temperature of the exhaust gas islikely to decrease, but the primary catalyst 54 is warmed up in a shorttime by the high-temperature exhaust gas from the exhaust pipe 51L, anda purification performance of the exhaust gas from the exhaust port 36Ris also improved. Thus, an early activation of the primary catalyst 54is implemented by intentionally adding a difference in the pipe lengthsto the exhaust pipes 51L and 51R.

The bent pipe 55 is formed in an L-shaped tubular shape in which avertical tubular portion 56 on an upstream side and a horizontal tubularportion 57 on a downstream side are connected. A cross-sectional shapeof the bent pipe 55 gradually changes from a circular shape to anelliptical shape from an upstream end toward a downstream end. Theprimary catalyst case 53 is connected to the upstream end of thevertical tubular portion 56 having a circular cross section, thesecondary catalyst case (catalyst case) 61 is connected to thedownstream end of the horizontal tubular portion 57 having an ellipticalcross section, and the exhaust gas that passes through the primarycatalyst 54 is guided to the secondary catalyst 62 on the lower side ofthe engine 30 by the bent pipe 55. A second oxygen sensor 84 is disposedon a wall surface of the horizontal tubular portion 57, and an oxygenconcentration of the exhaust gas that passes through the primarycatalyst case 53 is detected by the second oxygen sensor 84. A detectionresult of the second oxygen sensor 84 is used for feedback control ofthe fuel injection amount and diagnosis of catalyst deterioration.

As shown in FIGS. 3 and 4, the secondary catalyst case 61 is formed inan elliptical cylindrical shape, and is connected to the bent pipe 55 ina substantially horizontal posture. At this time, the secondary catalystcase 61 extends obliquely rearward from the left side toward a rightside (the one side to the other side in the engine width direction). Thesecondary catalyst 62 for purifying the exhaust gas that passes throughthe bent pipe 55 is accommodated in the secondary catalyst case 61. Thesecondary catalyst 62 is formed by adhering the catalyst substance to asurface of a honeycomb-shaped or lattice-shaped partition plate, andwhen the exhaust gas flowing in from the bent pipe 55 passes through thesecondary catalyst 62, the air pollutant reacts with oxygen and ispurified.

The chamber 71 is formed in an elliptical cylindrical shape, and isconnected to the secondary catalyst case 61 in a substantiallyhorizontal posture. A muffling chamber 72 (see FIG. 6) for reducing theexhaust noise is formed in the chamber 71. The chamber 71 extends in afront-rear direction, and an upstream end of the chamber 71 is joined toan outer wall surface of the secondary catalyst case 61 so as to coverthe outer wall surface of the secondary catalyst case 61. The chamber 71is disposed on a right side (the other side in the engine widthdirection) of a center line C2 of the engine 30 extending in thefront-rear direction. A tapered pipe 73 connected to a downstream end ofthe secondary catalyst case 61 and a punching pipe 74 connected to adownstream end of the tapered pipe 73 are provided inside the chamber71.

A cross-sectional shape of the tapered pipe 73 gradually changes from anelliptical shape to a circular shape from an upstream end toward thedownstream end. A large number of small holes are formed in a peripheralsurface of the punching pipe 74, and an inner side of the punching pipe74 and the muffling chamber 72 are connected to each other through thelarge number of small holes. When the exhaust gas enters the mufflingchamber 72 from the punching pipe 74, the exhaust gas is expanded in themuffling chamber 72, so that the exhaust noise is reduced. An outer wallof the chamber 71 has a double-cylinder structure, and a gap between aninner cylinder and an outer cylinder is filled with glass wool for soundabsorption. The chamber 71 is supported by the vehicle body frame 10 viaa bracket 75.

The exhaust pipe 81 is formed in a cylindrical shape and extendsrearward from a downstream end of the chamber 71. The muffler 82 (seeFIG. 1) is positioned on a right side of the rear wheel 23, and isconnected to a downstream end of the exhaust pipe 81. A muffling chamber(not shown) for reducing the exhaust noise is formed in the muffler 82,and a rear end of the muffling chamber is connected to the outsidethrough an exhaust port. Although a structure of the muffler 82 is notparticularly limited, one muffling chamber may be formed, or a pluralityof muffling chambers may be formed inside the muffler 82. Thus, theexhaust noise is reduced in two stages by the chamber 71 and the muffler82 in the exhaust device 50.

An arrangement configuration of the catalyst case and the chamber willbe described with reference to FIGS. 2 to 6. FIG. 5 is a cross-sectionalview taken along a line A-A of FIG. 4. FIG. 6 is a cross-sectional viewtaken along a line B-B of FIG. 4. In FIG. 5, the primary catalyst isomitted.

As shown in FIG. 2, the lower down tubes 15L and 15R branching from thelower end of the upper down tube 14 extend obliquely downward. Abranching point of the lower down tubes 15L and 15R are positioned belowthe exhaust ports 36L and 36R and above a height position O1 at a centerof the crankshaft 35. The oil filter 38 is provided on the front surfaceof the crankcase 31 in a V-shaped space between the lower down tubes 15Land 15R. The oil filter 38 is positioned on the center line C1 of theengine 30 extending in the upper-lower direction. Since the exhaustdevice 50 avoids a front of the oil filter 38, an entry path for a toolto the oil filter 38 is secured.

The oil pan 37 is attached to a lower surface of the crankcase 31. Abottom surface of the oil pan 37 includes a deep bottom portion 41 onthe left side (the one side in the engine width direction), and aninclined portion 43 which is inclined so that the bottom surface of theoil pan 37 becomes shallow from the deep bottom portion 41 toward theright side (the other side in the engine width direction) (see FIG. 6).The deep bottom portion 41 has a depth deeper than another portion ofthe bottom surface of the oil pan 37. A right side of the bottom surfaceof the oil pan 37 is recessed in an arch shape, and the exhaust device50 passes through a recess of the oil pan 37. Thus, on the lower side ofthe engine 30, a space for disposing the exhaust device 50 is formed ona right side of the deep bottom portion 41 of the oil pan 37. Apositional relationship between the exhaust device 50 and the oil pan 37will be described in detail later.

As shown in FIGS. 2 and 3, the primary catalyst case 53 is disposed infront of the lower down tube 15L. An upstream end of the primarycatalyst case 53 substantially coincides with the height position O1 atthe center of the crankshaft 35. The bent pipe 55 is bent in an L shapefrom the downstream end of the primary catalyst case 53 to the rightside, and a downstream end of the bent pipe 55 is positioned below theoil filter 38. Since the bent pipe 55 extends obliquely to reduce anoccupied area in the front and rear, the secondary catalyst case 61connected to the downstream end of the bent pipe 55 is compactlydisposed in the front. A rearmost portion of the downstream end of thesecondary catalyst case 61 substantially coincides with a front-rearposition O2 at the center of the crankshaft 35.

As shown in FIG. 4, the secondary catalyst case 61 is disposed betweenthe lower down tubes 15L and 15R so as to overlap the oil pan 37. Thesecondary catalyst case 61 extends obliquely rearward from the left sideto the right side, and the secondary catalyst case 61 intersects thecenter line C2 of the engine 30 extending in the front-rear direction.Since the secondary catalyst case 61 is obliquely disposed, an occupiedarea of the secondary catalyst case 61 in the front-rear direction canbe reduced, and an occupied area of the chamber 71 can be widely securedin a rear of the secondary catalyst case 61. The secondary catalyst 62accommodated inside the secondary catalyst case 61 is also obliquelydisposed similarly to the secondary catalyst case 61.

The chamber 71 extends in the front-rear direction, and is disposed onthe right side of the center line C2 of the engine 30 extending in thefront-rear direction. The upstream end of the chamber 71 is positionedin front of the front-rear position O2 at the center of the crankshaft35, and the downstream end of the chamber 71 extends to a bridge 16connecting the lower down tubes 15L and 15R. An entire length of thechamber 71 is set to a size that does not interfere with a center stand(not shown). The catalyst is not accommodated in the muffling chamber 72in the chamber 71, and the muffling chamber 72 is an expansion space ofthe exhaust gas. The chamber 71 functions as a primary muffler thatassists a muffling function of the muffler 82.

The upstream end of the chamber 71 is joined to the outer wall surfaceof the secondary catalyst case 61, and the muffling chamber 72 in thechamber 71 is widely secured to improve the muffling performance. Morespecifically, the chamber 71 is continuously connected to the secondarycatalyst case 61, and a space around the tapered pipe 73 (outside in aradial direction) disposed inside the chamber 71 is also used as themuffling chamber 72. The exhaust gas flows smoothly from the secondarycatalyst case 61 toward the chamber 71 by directly connecting thesecondary catalyst case 61 and the chamber 71 without interposinganother member such as a connecting pipe between the secondary catalystcase 61 and the chamber 71.

Thus, the primary catalyst case 53 is disposed in the front space of theengine 30, the secondary catalyst case 61 is disposed on the front partof the lower space of the engine 30, and the chamber 71 is disposed onthe rear part of the lower space of the engine 30. More specifically,most of the primary catalyst case 53 is disposed in front of the engine30 below the center of the crankshaft 35, and most of the secondarycatalyst case 61 is disposed on a front side of the center of thecrankshaft 35 (see FIG. 3). Then, most of the chamber 71 is disposed onthe lower side of the engine 30 and on a rear side of the center of thecrankshaft 35, and the muffling chamber 72 (see FIG. 6) having asufficient volume is secured on the rear part of the lower space of theengine 30 in the exhaust passage.

In the exhaust device 50, a required volume of the muffling chamber issecured by the chamber 71 and the muffler 82. Since the chamber 71serves as the primary muffler, a volume of the muffler 82 downstream ofthe chamber 71 can be reduced. In a state where a muffling performanceof the exhaust device 50 is maintained, a risk of heat damage is reducedby reducing a surface area of the muffler 82, and a degree of freedom indesigning of the muffler 82 is improved. In addition, the risk of theheat damage is reduced by intensively disposing the high-temperaturecatalyst in the vicinity of the engine 30. Further, a weight balance isoptimized by positioning a center of gravity of the secondary catalyst62 on the front side of the center of the crankshaft 35.

As shown in FIG. 5, the secondary catalyst case 61 is positioned insidean arch-shaped recess of the oil pan 37. The secondary catalyst case 61has an elliptical cross-sectional shape having a width larger than aheight of the secondary catalyst case 61. A major axis of the secondarycatalyst case 61 is oriented in a substantially horizontal direction,and a minor axis of the secondary catalyst case 61 is oriented in asubstantially vertical direction. An upper surface 63 of the secondarycatalyst case 61 faces a bottom surface 42 of the oil pan 37 in a widerange, and heat dissipation from the upper surface 63 of the secondarycatalyst case 61 is propagated to the bottom surface 42 of the oil pan37. In addition, a side surface 64 of the secondary catalyst case 61faces the inclined portion 43 of the oil pan 37, and heat dissipationfrom the side surface 64 of the secondary catalyst case 61 is propagatedto the inclined portion 43 of the oil pan 37.

As shown in FIG. 6, the chamber 71 is positioned inside the arch-shapedrecess of the oil pan 37. The chamber 71 has an ellipticalcross-sectional shape having a width dimension larger than a heightdimension. A major axis of the chamber 71 is oriented in a substantiallyhorizontal direction, and a minor axis of the chamber 71 is oriented ina substantially vertical direction. An upper surface 76 of the chamber71 faces the bottom surface 42 of the oil pan 37 in a wide range, andheat dissipation from the upper surface 76 of the chamber 71 ispropagated to the bottom surface 42 of the oil pan 37. In addition, aside surface 77 of the chamber 71 faces the inclined portion 43 of theoil pan 37, and heat dissipation from the side surface 77 of the chamber71 is propagated to the inclined portion 43 of the oil pan 37.

In a front view, the secondary catalyst case 61 overlaps the deep bottomportion 41 of the oil pan 37 (see FIG. 2). More specifically, thesecondary catalyst case 61 crosses a front of the deep bottom portion 41of the oil pan 37 (see FIG. 4), and heat dissipation of the secondarycatalyst case 61 is propagated to the oil pan 37 by running wind. Sincethe heat dissipation from the secondary catalyst case 61 and the chamber71 is propagated to the oil pan 37, the oil in the oil pan 37 isincreased to a suitable temperature in a short time. Since the minoraxes (heights) of the secondary catalyst case 61 and the chamber 71 aresmall, the engine 30 is low, vehicle body stability during traveling isimproved, and capacities of a fuel tank and an air cleaner are easilysecured.

Next, an arrangement configuration of the first and second oxygensensors will be described with reference to FIGS. 2 and 3.

As shown in FIGS. 2 and 3, the first oxygen sensor 83 is disposed in thecollecting pipe 52 in front of the engine 30. The first oxygen sensor 83is erected on the collecting pipe 52 in a state of facing the rear (anengine 30 side) between the exhaust pipes 51L and 51R. Since the firstoxygen sensor 83 is sandwiched between the exhaust pipes 51L and 51R andthe first oxygen sensor 83 is brought close to the engine 30, earlyactivation of the first oxygen sensor 83 is achieved by heat dissipationfrom the exhaust pipes 51L and 51R and the engine 30. When the exhaustgas from the exhaust pipes 51L and 51R is substantially uniform at adetection end of the first oxygen sensor 83, a detection accuracy of theoxygen concentration by the first oxygen sensor 83 is improved.

Since the first oxygen sensor 83 is disposed on a rear side of thecollecting pipe 52, the first oxygen sensor 83 is protected from aflying object from the front by the collecting pipe 52. Since the bentpipe 55 is present below the first oxygen sensor 83, the first oxygensensor 83 is protected from a flying object from below by the bent pipe55. The upper down tube 14 is positioned on a right side of the firstoxygen sensor 83, and the first oxygen sensor 83 overlaps the upper downtube 14 in a side view. Accordingly, since wiring is laid along theupper down tube 14, the wiring is easily connected to the first oxygensensor 83.

The second oxygen sensor 84 is disposed on the horizontal tubularportion 57 downstream of a bent portion of the bent pipe 55. The secondoxygen sensor 84 is erected on the horizontal tubular portion 57 in astate of facing upward (the engine 30 side). When the second oxygensensor 84 is brought close to the engine 30, early activation of thesecond oxygen sensor 84 is achieved by the heat dissipation from theengine 30. In the front view, the second oxygen sensor 84 overlaps thevertical tubular portion 56 upstream of the bent portion of the bentpipe 55. The second oxygen sensor 84 is protected from a flying objectfrom below by the horizontal tubular portion 57, and the second oxygensensor 84 is protected from a flying object from above by the verticaltubular portion 56.

The second oxygen sensor 84 is positioned between the lower down tubes15L and 15R, and the second oxygen sensor 84 overlaps the lower downtubes 15L and 15R in the side view. The second oxygen sensor 84 isprotected from a flying object from the left and right sides by thelower down tubes 15L and 15R. Since the second oxygen sensor 84 iscloser to the left side and wiring is laid along the lower down tube15L, the wiring is easily connected to the second oxygen sensor 84.Since the first and second oxygen sensors 83 and 84 are disposed on aleft side of the engine 30, a deviation of the first and second oxygensensors 83 and 84 in the left-right direction is reduced, and the wiringis easily collected.

The oil filter 38 is positioned on a right side of the second oxygensensor 84, and the second oxygen sensor 84 overlaps the oil filter 38 inthe side view. The second oxygen sensor 84 is protected from a flyingobject from the right by the oil filter 38. The oil filter 38 protrudestoward the front from the front surface of the crankcase 31, and thesecond oxygen sensor 84 is positioned in a rear of a front end of theoil filter 38. Accordingly, the second oxygen sensor 84 protrudingdirectly upward from the horizontal tubular portion 57 does not crossfront space of the oil filter 38, and the entry path for the tool to theoil filter 38 is secured.

As described above, according to the present embodiment, the secondarycatalyst case 61 is disposed on the front part of the lower space of theengine 30, and the lower space of the engine 30 is not occupied by thesecondary catalyst case 61. The chamber 71 is disposed in a wide rangeof the lower space of the engine 30, and the exhaust noise is reduced bythe muffling chamber 72 of the chamber 71. By securing the mufflingchamber 72 on an upstream side of the muffler 82, it is possible toreduce the size of the muffler 82. Therefore, in the state where themuffling performance is maintained, the risk of heat damage due to thesurface of the muffler can be reduced by reducing the size of themuffler 82, and the degree of freedom in designing the mufflerappearance can be improved.

In the present embodiment, the chamber is positioned on the right sideof the deep bottom portion of the oil pan, but a shape of the chamber isnot particularly limited. For example, as long as an attachment portionof the center stand can be relocated, as shown in a modification of FIG.7, a rear portion of a chamber 90 protrudes to a left side in a rearside of the oil pan 37, and the muffling chamber in the chamber 90 maybe further widened. In this case, the muffler on a side of the rearwheel can be eliminated to centralize a mass, the reduction of the riskof the heat damage due to the surface of the muffler, and an improvementof the degree of freedom in designing the muffler appearance can beachieved.

In the present embodiment, most of the secondary catalyst case isdisposed on the front side of the center of the crankshaft, but thesecondary catalyst case may be disposed in a range from the front spaceto the front part of the lower space of the engine. For example, thesecondary catalyst case may be disposed in the front space of theengine. Accordingly, by disposing the chamber in the wide range of thelower space of the engine, it is possible to further improve themuffling performance. The front part of the lower space of the enginerefers to a space on a front side of an intermediate position in thefront-rear direction in the lower space of the engine.

In the present embodiment, most of the chamber is disposed on the rearside of the center of the crankshaft, but the chamber may occupy atleast the rear part of the lower space of the engine. For example, thechamber may be disposed on the front side and the rear part of the lowerspace of the engine. The rear part of the lower space of the enginerefers to a space on a rear side of the intermediate position in thefront-rear direction in the lower space of the engine.

In the present embodiment, a press component may be used for a pipe ofthe exhaust device. The number of components and welding points can bereduced.

In the present embodiment, the left side of the bottom surface of theoil pan is formed at the deep bottom, but a part of the bottom surfaceof the oil pan may be formed at the deep bottom. For example, if theexhaust device does not interfere with the oil pan, the right side ofthe bottom surface of the oil pan may be formed at the deep bottom.

In the present embodiment, the upstream end of the chamber is joined tothe outer wall surface of the secondary catalyst case, but the chamberand the secondary catalyst case may be separated from each other, andthe chamber and the secondary catalyst case may be connected to via aconnecting pipe.

In the present embodiment, the primary catalyst case and the secondarycatalyst case are provided in the exhaust device, but at least onecatalyst case may be provided in the exhaust device.

In the present embodiment, the engine is the parallel two-cylinderengine, but a type of engine is not particularly limited, and forexample, the engine may be a single-cylinder engine.

In the present embodiment, the cross-sectional shape of the secondarycatalyst case and the cross-sectional shape of the chamber are formed inan elliptical shape, but the cross-sectional shape of the secondarycatalyst case and the cross-sectional shape of the chamber may be formedin a cross-sectional shape having the width larger than the height.Depending on a shape of the frame, a shape of a bottom surface of theengine, and a minimum ground clearance, the cross-sectional shape of thesecondary catalyst case and the cross-sectional shape of the chamber maybe formed in a circular shape.

In the present embodiment, the secondary catalyst case crosses the frontof the deep bottom portion of the oil pan, but a positional relationshipbetween the secondary catalyst case and the deep bottom portion of theoil pan is not particularly limited. When the heat dissipation from thesecondary catalyst case is propagated to the oil pan by the runningwind, the secondary catalyst case may overlap the deep bottom portion ofthe oil pan in the front view.

In the present embodiment, the secondary catalyst case extends obliquelyrearward from the left side to the right side, but the secondarycatalyst case may extend in the front-rear direction.

In the present embodiment, the oxygen sensor is illustrated as a gassensor, but the gas sensor may be any sensor that can detect an averagecharacteristic of the exhaust gas, and may be, for example, an exhaustnoise sensor that detects the exhaust noise of the exhaust gas.

The exhaust device of the present embodiment is not limited to theengine of the above straddle-type vehicle, but may be adopted for anengine of another type of straddle-type vehicle. The straddle-typevehicle is not limited to a motorcycle, and may be any vehicle on whichan engine is mounted. The straddle-type vehicle is not limited togeneral vehicles on which a driver rides in a posture of straddling aseat, and includes a scooter-type vehicle on which the driver rideswithout straddling the seat.

As described above, the exhaust device (50) of the present embodiment isan exhaust device that guides the exhaust gas from the exhaust pipe(51L, 51R) in front of the engine (30) to the muffler (82) in the rearof the engine, and includes the catalyst case (secondary catalyst case61) in which the catalyst (secondary catalyst 62) that purifies theexhaust gas that passes through the exhaust pipe is accommodated; andthe chamber (71) in which the muffling chamber (72) that reduces theexhaust noise is formed downstream of the catalyst case, in which thecatalyst case is disposed in the range from the front space to the frontpart of the lower space of the engine, and the chamber is disposed so asto occupy at least the rear part of the lower space of the engine.According to this configuration, the catalyst case is disposed on thefront space or the front part of the lower space of the engine, and thelower space of the engine is not occupied by the catalyst case. Thechamber is disposed in the wide range of the lower space of the engine,and the exhaust noise is reduced by the muffling chamber of the chamber.By securing the muffling chamber on the upstream side of the muffler, itis possible to reduce the size of the muffler. Therefore, in the statewhere the muffling performance is maintained, the risk of the heatdamage due to the surface of the muffler can be reduced by reducing thesize of the muffler, and the degree of freedom in designing the mufflerappearance can be improved.

In the exhaust device of the present embodiment, the front space of theengine is in front of the engine below the center of the crankshaft(35), the front part of the lower space of the engine is on the lowerside of the engine and on the front side of the center of thecrankshaft, and the rear part of the lower space of the engine is on thelower side of the engine and on the rear side of the center of thecrankshaft. According to this configuration, the chamber can be disposedin the wide range of the lower space of the engine by an amount that thecatalyst case is disposed on the front side of the center of thecrankshaft.

In the exhaust device of the present embodiment, the upstream end of thechamber is positioned on the front side of the center of the crankshaft.According to this configuration, the muffling chamber of the chamber canbe widely secured.

In the exhaust device of the present embodiment, the upstream end of thechamber is joined to the outer wall surface of the catalyst case.According to this configuration, it is possible to extend the upstreamend of the chamber to the catalyst case to secure a wide mufflingchamber. The exhaust gas from the catalyst can flow smoothly into thechamber.

In the exhaust device of the present embodiment, the catalyst caseextends obliquely rearward from the one side toward the other side inthe engine width direction. According to this configuration, since thecatalyst case is obliquely disposed, the occupied area of the secondarycatalyst case in the front-rear direction can be reduced, and theoccupied area of the chamber can be widely secured in the rear of thecatalyst case.

In the exhaust device of the present embodiment, the catalyst case andthe chamber have the cross-sectional shape having the width larger thanthe height. According to this configuration, the engine can be disposedlow to improve the vehicle body stability during traveling.

In the exhaust device of the present embodiment, the oil pan (37) thatstores the oil is provided at a lower portion of the engine, and theupper surfaces (63, 76) of the catalyst case and the chamber face thebottom surface (42) of the oil pan. According to this configuration, theheat dissipation from the upper surfaces of the catalyst case and thechamber can be propagated to the bottom surface of the oil pan, and theoil in the oil pan can be increased to the suitable temperature in theshort time.

In the exhaust device of the present embodiment, a part of the bottomsurface of the oil pan is formed at the deep bottom, and the catalystcase overlaps the deep bottom portion (41) of the oil pan in the frontview. According to this configuration, the heat dissipation from thecatalyst case can be propagated to the oil pan by the running wind, andthe oil in the oil pan can be increased to the suitable temperature inthe short time.

In the exhaust device of the present embodiment, the catalyst casecrosses the front of the deep bottom portion. According to thisconfiguration, the heat dissipation from the catalyst case can be mostlypropagated to the oil pan by the running wind, and the oil in the oilpan can be increased to the suitable temperature in the short time.

In the exhaust device of the present embodiment, the deep bottom portionis formed on one side in the engine width direction of the bottomsurface of the oil pan, and the bottom surface of the oil pan isinclined so as to become shallow from the deep bottom portion toward theother side in the engine width direction, and the side surfaces (64, 77)of the catalyst case and the chamber face the inclined portion (43) ofthe bottom surface of the oil pan. According to this configuration, theheat dissipation from the side surfaces of the catalyst case and thechamber can be propagated to the inclined portion of the bottom surfaceof the oil pan, and the oil in the oil pan can be increased to thesuitable temperature in the short time.

Although the present embodiment has been described, the above-describedembodiment and modification may be combined entirely or partially asanother embodiment.

The technique of the present disclosure is not limited to theabove-described embodiment, and various changes, substitutions, andmodifications may be made without departing from the spirit of thetechnical idea of the present disclosure. Further, the present inventionmay be implemented using other methods as long as the technical idea canbe implemented by the methods through advance of the technology or otherderivative technology. Accordingly, the claims cover all embodimentsthat may be included within the scope of the technical concepts.

What is claimed is:
 1. An exhaust device, configured to guide exhaustgas from an exhaust pipe in front of an engine to a muffler in a rear ofthe engine, the exhaust device comprising: a catalyst case in which acatalyst configured to purify the exhaust gas that passes through theexhaust pipe is accommodated; and a chamber in which a muffling chamberconfigured to reduce an exhaust noise is formed downstream of thecatalyst case, wherein the catalyst case is disposed in a range from afront space of the engine to a front part of a lower space of theengine, and the chamber is disposed so as to occupy at least a rear partof the lower space of the engine.
 2. The exhaust device according toclaim 1, wherein the front space of the engine is in front of the engineand below a center of a crankshaft, the front part of the lower space ofthe engine is on a lower side of the engine and on a front side of thecenter of the crankshaft, and the rear part of the lower space of theengine is on the lower side of the engine and on a rear side of thecenter of the crankshaft.
 3. The exhaust device according to claim 2,wherein an upstream end of the chamber is positioned on the front sideof the center of the crankshaft.
 4. The exhaust device according toclaim 1, wherein the upstream end of the chamber is joined to an outerwall surface of the catalyst case.
 5. The exhaust device according toclaim 1, wherein the catalyst case extends obliquely rearward from oneside toward another side in an engine width direction.
 6. The exhaustdevice according to claim 1, wherein the catalyst case has across-sectional shape having a width larger than a height, and thechamber has a cross-sectional shape having a width larger than a height.7. The exhaust device according to claim 1, wherein an oil panconfigured to store oil is provided at a lower portion of the engine,and an upper surface of the catalyst case and an upper surface of thechamber face a bottom surface of the oil pan.
 8. The exhaust deviceaccording to claim 7, wherein the bottom surface of the oil pan includesa deep bottom portion having a depth deeper than another portion of thebottom surface, and the catalyst case overlaps the deep bottom portionof the oil pan in a front view.
 9. The exhaust device according to claim8, wherein the catalyst case crosses a front of the deep bottom portion.10. The exhaust device according to claim 8, wherein the deep bottomportion is formed on the one side in the engine width direction of thebottom surface of the oil pan, and the bottom surface of the oil panincludes an inclined portion which is inclined so as to become shallowfrom the deep bottom portion toward the other side in the engine widthdirection, and a side surface of the catalyst case and a side surface ofthe chamber face the inclined portion of the bottom surface of the oilpan.